truncate remainingSatoshis to match lamassu-machine amount exactly

lib/postgresql_interface.js

@@ -15,6 +15,8 @@ function inspect(rec) {
 }
 */
 
+var SATOSHI_FACTOR = 1e8;
+
 function isUniqueViolation(err) {
   return err.code === '23505';
 }
@@ -170,10 +172,34 @@ function billsAndTxs(client, session, cb) {
   });
 }
 
+
+// TODO: these should broken out to a common library, they are used by both
+// lamassu-machine and lamassu-server and need to stay in sync.
+function bitcoinFractionalDigits(amount) {
+  var log = Math.floor(Math.log(amount) / Math.log(10));
+  return (log > 0) ? 2 : 2 - log;
+}
+
+function truncateBitcoins(bitcoins) {
+  var decimalDigits = bitcoinFractionalDigits(bitcoins);
+  var adjuster = Math.pow(10, decimalDigits);
+  return (Math.round(bitcoins * adjuster) / adjuster);
+}
+
+function truncateSatoshis(satoshis) {
+  var bitcoins = satoshis / SATOSHI_FACTOR;
+  var truncated = truncateBitcoins(bitcoins);
+  return Math.round(truncated * SATOSHI_FACTOR);
+}
+// END TODO
+
 function computeSendAmount(tx, totals) {
+  var fiatRemaining = (tx.fiat || totals.billsFiat) - totals.txFiat;
+  var satoshisRemaining = (tx.satoshis || totals.billsSatoshis) -
+    totals.txSatoshis;
   var result = {
-    fiat: (tx.fiat || totals.billsFiat) - totals.txFiat,
+    fiat: fiatRemaining,
-    satoshis: (tx.satoshis || totals.billsSatoshis) - totals.txSatoshis
+    satoshis: truncateSatoshis(satoshisRemaining)
   };
   if (result.fiat < 0 || result.satoshis < 0) {
     logger.warn({tx: tx, totals: totals, result: result},